It has been long known that paint coatings could be made electrically conductive by the inclusion therein of particulates comprised of matter having substantial electrical conductivity properties. A number of paints are known incorporating electrically conductive metal particles or particles of carbon therein which can function to conduct static electricity, for example, accruing in a structure which the paint is protecting.
More recently, with the advent of elaborate solid state circuitry devices the requirements for performance characteristics of conductive coatings have become more stringent. These solid state devices include electronic memory devices susceptible to so-called EMI (electromagnetic wave interference) or RFI (radio frequency interference). EMI and RFI have demonstrated a capability for deprogramming memory devices and a potential for ruining circuitry thus rendering complicated electronic circuitry devices useless. In the past, EMI or RFI sensitivity was not of particular concern to the electronics industry as electronics devices typically were housed in metallic housings which functioned at least in part also to shield electronic devices from EMI or RFI interference.
More recently, electrically non-conductive (relatively) plastics have found increasing use in forming cabinetry for containing electronic components and, typically, such plastics are sufficiently electrically non-conductive whereby, unlike metal housings, no significant shielding from RFI or EMI electromagnetic radiation is achieved.
Electrically conductive paints have therefore achieved a certain acceptance in imparting electromagnetic radiation protection to electronic circuitry contained in plastic cabinetry. Such conductive paint coatings are applied typically to interior surfaces of cabinetry encapsulating electronic components and then are grounded in suitable or conventional fashion to dispose properly of electrical energy generated by impinging electromagnetic radiation.
Such paint coatings configured for protecting electronic devices from EMI and RFI electromagnetic radiation must provide substantial conductivity to be effective. While various entities within the electronics industry have varying standards for measuring the conductivity performance for such coatings, it would appear that a generally universal performance standard for EMI/RFI cabinetry protection of about 1 .OMEGA./square after aging would appear to be emerging. Particularly in the range of 30 megahertz to 1 gigahertz frequencies an attenuation of at least 30 db is considered adequate for most shielding application in a shielding coating. A 30 db reduction requires approximately a 1.3-1.5 .OMEGA./square value in the shielding coating.
It is known to formulate coatings including electrically conductive additives such as carbon or metals in particulate form where the coatings have a solvent base. More recently, coatings have been formulated having a water base and including therein electrically conductive particulates of, for example, carbon or metal particles.
A number of factors influence the acceptability of a particular coating or paint for use on a paint spraying line such as may be associated with the manufacture of cabinetry for electronics components, having an applied EMI/RFI paint coating upon inner surfaces of the cabinetry. Desirably, such coatings:(i) should be air dryable rather than requiring oven drying; (ii) should be substantially free from a tendency for rapid settling of any included particulates whereby constant or frequent agitation of the coating during application procedures on the spraying line is not necessary and whereby upon shutdown of a production spray line for an extended period such as over a weekend, settling in spray hoses, nozzles and paint containers will not cause deleteriously affect spray line operations upon startup; (iii) should have good properties of adhesion to the plastic cabinetry to which it is applied; (iv) should have outstanding storage stability; (v) especially with reference to desirable settling characteristics of any particulates therein; and in addition (vi) should be substantially resistant to the effects of abrasion, humidity aging, and temperature cycling once applied.
Particularly, solvent based conductive paints traditionally have included a significant quantity of included solvents which evaporate upon paint drying. The size and cost of physical equipment necessary to cope adequately with recovery of such solvents on a commercial scale, for reasons such as the prevention of air pollution or solvent reuse, can detract significantly from the desirability of using solvent based paint coatings in effecting EMI/RFI protection for electronics hardware cabinetry. Emerging statutory/regulatory postures could exclude the use of solvent based paints.
Water based coatings, often also known as latex paints, tend to evolve relatively small quantities of solvent upon drying but are more prone to difficulties with:(i) adhesion, particularly to the surfaces of certain plastics such as NORYL.RTM., or LEXAN.RTM. (General Electric Co.), CALIBRE.TM. or STYRON.RTM. (Dow), CYCOLAC.RTM. (Borg Warner), or TEMPRITE.RTM., FIBERLOC.RTM. or GEON.RTM. (B.F. Goodrich); (ii) settling problems while in use on a spray line; and (iii) a final conductivity of the coating being undesirably substantially greater than the preferred maximum of 1 .OMEGA./square. Particularly, those water based paints employing metal particles or flakes and most particularly nickel particles or flakes as the conductive particulate within the paint coating have tended to demonstrate undesirable settling characteristics and, to a considerable extent, substantially undesirable electrical conductance properties unless aged for a considerable time period, usually weeks, before application.
A water based, electrically conductive, air drying water based paint having acceptable settling and aging characteristics would have substantial application in the electronics industry, particularly where the coating resulting from application of such a paint to a plastic electronics cabinet component is a coating having an electrical conductance parameter of 1 .OMEGA./square or less where such paints or coatings can be applied and be effective immediately after manufacture.